It is known in the art to produce felts by applying pressure and heat to a tangle of threads. It has also long been known to produce multi-ply felt by pressing bats of fleeces together, for example bats of differing quality as taught by U.S. Pat. No. 762,264 to Waring.
At least since the early 1990s thin webs of synthetic fibers have been produced by a technique known as extrusion fiber melt blowing. The process basically involves extruding multiple strands through a line of holes in a fiber melt blowing die, and allowing the parallel strands to tangle while molten, to provide cohesion between the strands when cooled, forming the web.
Extrusion fiber melt blowing produces molten strands having a distribution of thicknesses that become randomly tangled. As the strands are contact welded at tangle contact points, the resulting web generally has lower strength in one direction, i.e. the direction orthogonal to the direction of extrusion. There are many applications of melt blown webs that are applied to backings or other layers formed differently.
U.S. Pat. No. 6,048,808 to Kurihara et al. teaches a nonwoven fabric of stretched filaments of polymers of different kinds, having a strength equal to that of a woven fabric, and a method for manufacturing the same. The fabric is characterized in that the nonwoven fabric is provided with stretched filament webs comprising long filaments formed out of a plural kinds of thermoplastic polymers of different properties, the long filaments as a whole being aligned in one direction. The fabric may be composed of threads that are spunbonded or melt-blown. Kurihara et al. teach that a nonwoven fabric can be obtained by laminating webs of different aligned directions. Both crosswise and obliquely intersecting ways are applicable to laminating either longitudinally aligned webs or transversely aligned webs. Kurihara et al. require webs of different kinds of polymers be used, as does lamination, the method of binding the webs, and the product resulting from the teachings of Kuirhara et al. does not have the useful properties described in the present invention.
U.S. Pat. No. 5,891,482 teaches a melt blowing apparatus for producing a layered filter media web product. A die apparatus wherein a layered web of melt blown fibrous filter media is produced by a unitary die including several die sources with facing layers of the fibrous filter media being attenuated by opposed fluid streams at preselected included angles and with the fiber layers being free from bonding together and with the fibers in each layer being minimally bonded.
United States patent application publication number 2004/0035095 to Healey teaches a non-woven filter composite and a method for forming the composite. According to Healey, a cost effective, high efficiency, low pressure drop, adsorptive, non-woven filter media is provided comprising a high surface area synthetic microfiber, e.g., melt blown, fine fiber layer. The filter media can also include one or more non-woven spun bond layers and can be combined with a coarse fiber support layer. The coarse fiber support layer can itself be a low pressure drop synthetic microfiber, e.g., melt blown, layer adhered to a spunbond layer, and can serve as a prefilter to enhance overall performance.
Accordingly, it is known to produce a composite having a plurality of different non-woven layers.
Furthermore as taught in Journal of Materials Science 40 (2005) 2675-2677 entitled “How to design a structure able to mimic the arterial wall mechanical behavior” to Jouan et al., it is known to produce non-woven mats of polypropylene fibers (6-14 μm in diameter), by applying a heat treatment (15 min. at 120-150° C.) to a stack of 20 melt blown webs. Stress-strain curves of the mats produced by different temperatures of the heat treatment were compared with that of a femoral artery.
There remains a need for a non-woven mat that is inexpensive to produce, and has useful mechanical properties, including a uniform porosity.